Use permalink if exists instead append .html

Hi mates,

just another PR. Actually the compilation process when someone put a `permalink` strategy the `.html` is appended by default. This trend could be a problem if I need to create particular pages for eg: `permalink: /installer` that is a script file.

I think that if place a permalink Phrozn has to use my permalink and not append `.html`, of course if I create a permalink as `/create/this/path/` the result is `/create/this/path/index.html` 

What do you think about this?

Bye